This invention relates generally to acid catalyzed processing of hydrocarbons and, more specifically, to increasing the dissolved H.sub.2 O content in such processing.
A wide variety of reactions are catalyzed or otherwise promoted by substances generally classified as acids. The alkylation of isoparaffins with olefins is one such reaction.
High octane material can be produced by reacting an isoparaffin such as isobutane with C.sub.2 -C.sub.5 olefins, forming branched chain isoparaffinic hydrocarbons in the gasoline boiling range. Such processing is commonly referred to as alkylation. Alkylation can be done under both thermal and catalytic conditions. Thermal alkylation typically requires high temperatures (about 500.degree. C. or more) and pressures (about 150 to 300 atm). Catalytic alkylation, in contrast, generally occurs at significantly lower temperatures and pressures. While dependent on the catalyst used in such processing, the temperatures used are usually in the range of from about -30.degree. to about 100.degree. C. and the pressure need only be sufficient to maintain the reactants in the liquid phase.
The catalytic alkylation of an isoparaffin with one or more olefins to produce a branched chain paraffin, using hydrofluoric acid (HF) as a catalyst, is a commercially important process for producing high octane gasoline.
Depending on the olefin used in alkylation, the alkylate octanes are usually 90 to 96 RON (Research Octane Number) and 88 to 94 MON (Motor Octane Number). The production of high, clear octane makes alkylate a desirable feedstock for gasolines. In addition, alkylate is typically nontoxic, has a high heat of combustion, and is good for both high and low-speed driving.
Butylenes are a preferred alkylation olefinic feed because they typically produce the highest octane alkylate at the least cost with common catalyst. Propylene and amylenes (pentenes) can also be readily alkylated. Ethylene is rarely alkylated. Alkylation feed is typically charged directly from a catalytic cracking and/or a coking unit. Additional isobutane, other than the amount present in these olefinic feeds, is commonly required for the alkylation process. Sources for such additional isobutane may include the crude unit, reformer, hydrocracker, isomerate and purchased field butanes, for example.
Products of the process may typically include alkylate, LPG (Liquified Petroleum Gas) and normal butane. Existing and potential environmental/health regulations limiting lead, olefins and/or aromatics in gasoline may, in the future, increase the need for alkylation to maintain refinery pool octanes.
Catalytic alkylation processing was first commercialized using a sulfuric acid catalyst. This was followed shortly thereafter by the first alkylation unit to use hydrofluoric acid (HF) as an acid catalyst for alkylation.
In general, in alkylation processes, an alkylatable material, e.g., an isoparaffin, preferably isobutane, for example, is reacted with an alkylating agent, e.g., an olefin, preferably a light olefin such as propylene, butylenes or amylenes. The feed typically consists of several olefins and paraffins, while the alkylate product contains numerous compounds in the C.sub.5 to C.sub.12 range. All true reaction products are at least singly branched. (Note: Some feed components are unbranched, nonreactive n-paraffins which become part of the alkylate. Hydrogen-transfer reactions also produce normal paraffins.) Normal paraffins are typically also present in the feed and generally cannot be economically eliminated prior to reaction. For the most part, however, such normal paraffins are unaffected in the process.
Two liquid phases typically exist in the alkylation reactor because alkylate and the isoparaffin, e.g., isobutane, are only sparingly soluble in the acid phase. However, the alkylation reaction is thought to generally occur in the acid phase.
In addition, while isobutane is only slightly soluble in the acid (about 3% in hydrofluoric acid), the olefins are essentially infinitively soluble in the acid in the form of an ester. Therefore, high isobutane to olefin ratios must be used to assure that the alkylation rather than polymerization reactions occur. High mixing intensities are also required so that the isobutane lost by reaction from the acid phase can be replenished and olefins will be widely dispersed to minimize polymerization.
While commercial isoparaffin-olefin alkylation is normally catalyzed by sulfuric or hydrofluoric acid, neither of these acid catalysts can conveniently be used to alkylate ethylene, because, in such processing, ethylene merely forms stable esters.
Hydrofluoric acid alkylation generally occurs at about 75.degree. to about 100.degree. F. in commercial units, while the sulfuric acid process typically operates at about 30.degree. to about 65.degree. F. Hydrofluoric acid can perform well at a higher temperature because of its greater isobutane solubility. This property in combination with the much lower density of hydrofluoric acid as compared to sulfuric acid, allows hydrofluoric acid systems to typically operate without requiring the use of reactor mixers. In addition, hydroflouric acid unit reactor temperatures can generally be maintained by using normally available cooling water and controlling olefin flow rate. In contrast, the sulfuric acid-catalyzed process temperature is limited by increased acid viscosity and possible acid freezing at the lower bound and acid degeneration due to oxidation at the upper bound. Thus, using a hydrofluoric acid as opposed to a sulfuric acid catalyst process can save refrigeration system costs.
In such alkylation processing, although the acid is a catalyst, it is "used" by reaction with contaminants and diluted with polymerization products. Because acidity must be maintained above a minimum strength to avoid rapid acidity decay (due to polymerization being increasingly favored over alkylation as acidity declines), fresh acid typically must periodically be added in the process.
Wet HF acid, however, can be extremely corrosive and consequently feed streams in HF alkylation units are typically dried prior to use. Standard HF alkylation unit designs include wet feed stream driers (commonly designed to result in the stream passing therethrough to exist containing no more than about 20-50 ppm of water therein) to guard against high water concentrations and the presence of entrained or "free" water, which can contribute to increased corrosion and acid consumption. For example, treatment of the specified stream with bauxite or molecular sieves is commonly used for such drying. Water leakage past the driers, however, concentrates in the acid and is subsequently removed from the process system along with the acid soluble oil (ASO), via the acid rerun tower bottoms. The amount of feedstock impurities will typically govern the frequency and severity of acid rerun tower operation and hence the equilibrium water content of the HF acid. Thus, when using a relatively poor quality of fresh hydrocarbon feed, the H.sub.2 O content of the acid will, over time, be significantly reduced.
The presence of a moderate level of water in the acid phase, however, has been reported to have a beneficial effect on the alkylate composition produced in hydrofluoric acid units. Hutson and Hays of Phillips Petroleum Company, in "Reaction Mechanism for Hydrofluoric Acid Alkylation," ACS Symp. Ser., 55 (Ind. Lab, Alkylations), pp. 27-56 (1977), have shown that increasing the water in acid from 0.25% to about 2.8% decreases the formation of low octane C.sub.9 + material. The authors have also identified that proper water control may be significant in order to maximize alkylate quality and yield.
In view of the beneficial octane effects achieved through maintaining the H.sub.2 O content of the acid within a specified range and the known corrosive effects of undesirably high water concentrations and the presence of entrained water in HF acid, a process and a system for the safe addition of H.sub.2 O is needed.
Over the years various processes and equipment have been suggested in patents for catalytic alkylation using HF acid catalyst. Included in these patents are U.S. Pat. Nos. 4,276,439 and 4,383,977 which disclose that the hydrogen fluoride catalyst is generally 85 to 98 wt. % HF and 2 to 15 wt. % water, acid soluble oils and hydrocarbons; U.S. Pat. Nos. 4,220,806 and 4,225,737 which disclose that the alkylation catalyst employed therein generally contains about 75 wt. % or more of titratable acid, about 5 wt. % or less of water and the remainder constituting organic diluent, with a particular preferred catalyst comprising about 85 wt. % hydrofluoric acid and less than about 1 wt. % water; U.S. Pat. No. 4,214,114 which discloses that it is possible to use hydrofluoric acid containing as much as about 10% water; U.S. Pat. No. 4,207,423 which discloses that the water content of the catalyst phase should be between about 0.5 and 5.0 wt. %, preferably below about 2 wt. % of the total catalyst phase and still more preferably below about 1.5 wt. % of the total catalyst phase; U.S. Pat. No. 3,726,940 which identifies 2 wt. % water as the optimum water concentration of the fluid stream flowing to the conduit into the rerun column; and U.S. Pat. No. 2,570,574 which identifies that the hydrogen fluoride catalyst may contain minor quantities (up to 5%-10%) of water although substantially anhydrous hydrogen fluoride is preferred.
None of these patents show or suggest a process or system for a safe addition of H.sub.2 O to an HF alkylation system.